The present invention concerns fusable compositions of the type which are utilized to cover the surface of liquid steel in the mold during a continuous casting process.
The more recent use in metallurgical operations of installations permitting to produce in continuous processes solid products such as slabs, sheets, etc. starting from liquid steel has brought with it a number of specific problems arising in this type of casting processes. In particular it has been found indispensable to isolate the liquid metal which continuously is present at the surface of the mold from the atmosphere in order to prevent its rapid cooling. To accomplish this protection one has resorted to powders, so-called covering powders of which the melting point is below the temperature of the liquid steel so that they melt and cover the metal with a uniform layer. Apart from its principal object to assure the thermal protection of the metal, these products have soon found other important functions such as the protection of the metal against oxidation, the absorption and dissolving of inclusions which flow up and finally the lubrication between the mold and the skin of the solidified metal which comes in contact with the walls of the mold.
In order to truly accomplish these various functions it is essential that the powder form a liquid homogeneous phase by melting upon contact with the metal. In addition the conditions of the casting process and the nature of the steel require that the liquid thus formed must have specific physical and chemical properties and in consequence it is necessary that the solid product initially present should have an overall composition which is equally perfectly defined.
A particularly satisfactory solution consists in preparing by synthesis a glass which has the desired composition. In the practical application it is for economical reasons preferred in certain cases to rather have powders which are prepared by mixture of the various components. It will be understood that the selection of the components of the mixture is particularly impartant if one wants to obtain a solid product or "pulverulent cover" which fulfills completely the different functions which are required.
The object of the present invention is therefore to provide for a pulverulent product which is well suited for the surface protection of liquid steel.